1. Technical Field
The present invention relates generally to a trellis-based slicer, and, more specifically, to a soft trellis slicer that improves the performance of a decision-directed phase tracker in a high definition television receiver.
2. Description of Related Art
Since the invention of television in the 1920's the resolution of a television picture has been limited by the encoding standard for the television signal. With advances in broadcasting, signal processing and television equipment, these standards have become increasingly obsolete. In the 1990's, the United States Federal Communication Commission (“FCC”) announced that a slice of the frequency spectrum would be allocated for high definition television (“HDTV”). HDTV signals provide a clearer picture and higher resolution than that of the current analog NTSC (“National Television Standards Committee”) or PAL (used primarily in Europe) television signals.
A number of television hardware, broadcast and computer companies in the United States began to negotiate the establishment of new standards for the broadcasting HDTV. These companies, known as the Grand Alliance, understood that encoding standards and radio frequency (“RF”) modulation standards would have to be developed and agreed to before HDTV could become commercially viable. The Grand Alliance companies knew that competing standards might divide both the television and the broadcasting markets, as competing standards had split the market during the development of the video cassette recorder (“VCR”), that is, with the simultaneous development of the Betamax and VHS Systems.
In 1994, the Grand Alliance adopted and, in 1995, the FCC agreed to (“Moving Picture Experts Group”) MPEG II encoding system and a Vestigial Sideband (“VSB”) system for the transmission of HDTV signals. The signal protocol adopted was proposed by the Advanced Television Systems Committee (“ATSC”). Within the new standard, two transmission modes were established, a “terrestrial” mode for broadcast transmission and a “cable” mode for non-broadcast transmission. The terrestrial mode employs an 8-level modulation scheme known as 8VSB, and the cable mode employs a similar 16-level modulation scheme termed 16VSB. The 16VSB scheme can be used in non-broadcast transmission because a higher signal-to-noise ratio exists in non-broadcast transmission than is found in broadcast transmission. The use of 16VSB enables a non-broadcast transmitter to send two channels in the 6 MHz bandwidth allocated for each transmitter by the FCC, as opposed to one channel in the 8VSB terrestrial mode.
There were several technical hurdles that needed to be overcome before HDTV could be made publicly available. Perhaps one of the most significant hurdles was that new equipment, such as transmitters and receivers, had to be designed to generate and process these new types of signals. Over the last several years, a number of standard designs have been proposed to handle ATSC signals. While some basic technical questions have been resolved, other specific problems remain.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.